The invention is in the field of pultruded composites, a field in which the inventors have been working for years, and each of the inventors has a number of patents to his credit. Pultruded composites, that is, elongated members formed by pulling bunched fibers such as glass fibers through a resin bath and then through a die, have been used most extensively to form simple stick-like products. For example, many ax handles and hammer handles are pultrusions, and the number of pultruded reflective highway delineating posts must be in the millions. A utility pole of composite construction is illustrated in U.S. Pat. No. 4,803,819, issued Feb. 14, 1989.
The structural members which this disclosure details could be made from a wide variety of reinforcement fibers and matrix binders. The fibers could be glass, Kevlar.TM. or carbon, to name just three, and the matrix could be a thermoplastic such as polypropylene or a thermoset resin such as a polyester, a vinylester, or epoxy.
By their nature, pultruded composites have a potential for use in a great many fields and products. The nature of the resin and the fibers that are used in any particular pultrusion may be almost infinitely varied to produce wide variations in such different characteristics as modulus of elasticity, electrical or heat conductivity or resistance, resistance to ultraviolet light, resistance to the aging effects of certain chemicals, and so forth.
By the nature of the manufacturing process, pultrusions can be made in an unlimited variety of shapes, from a simple hollow or solid stick or beam to a configuration with a complex cross-section created by a combination of dies and mandrels. Although all manufacturing processes require some maintenance, with pultrusions, once the initial die is made, the pultrusion can be made continuously twenty-four hours a day with little expense other than the direct materials and production costs. Obviously, this process is ideal for producing elongated members that have complex cross-sections but which are used on an ongoing and virtually unlimited basis.
Pultrusions are also characterized by having a relatively high strength and moderate cost when glass fibers are used, and having a good strength-to-weight ratio when configured in appropriate cross-sections.
Compared to steel, which is the most common structural material in use, pultruded composites offer decided advantages as enumerated above, including virtual immunity to corrosion, non-conductivity, and in many applications about the same strength-to-weight ratio and cost-to-strength ratio. For these reasons, there have been efforts over the last few years on the part of the instant inventors and others to move the use of structural composites more into the mainstream where their advantages may be realized to a fuller extent. For example, one of the instant inventors has built vehicle-carrying railroad cars virtually completely from composites.
There is an area in which composites must be treated quite differently from steel. When steel is bolted, as is usually the case in field assembly, the bolt holes reduce the net area at the joint. Nontheless, bolt holes can be placed close together without concerns of tearout, or bearing strength failure. Thus, angled steel towers may have 10 or more bolts at a joint face, closely spaced to each other. Because of low bearing strength in composites, bolts cannot be spaced nearly as closely as in steel construction and bearing area becomes a critical design issue.
Consequently there is a need for effective coupling means between pultruded composite structural members. Coupling means need be provided to couple members of different cross-sectional shapes and sizes which meet one another at different angles in three dimensions. The luxury of being able to position a flange or produce a bolt hole virtually anywhere on the member, inherent in the use of steel, does not exist for composites. The challenge, therefore, is to create coupling means which are as orientationally versatile as possible and capable of joining two widely different member configurations, while at the same time being as strong, light-weight, compact and inexpensive as possible.